Study on Microstructural Deformation of Working Sn and SnSb Anode Particles for Li-Ion Batteries by in Situ Transmission X-ray Microscopy

Sn-containing compounds are potential high-capacity anode materials for Li-ion batteries. They, however, suffer from significant dimensional variations during electrochemical lithiation and delithiation, causing cycling instability. Understanding the dynamics of these deformation processes may provide valuable information in the establishment of viable high-energy anodes. In this paper, the evolution of interior microstructures of two types of Sn-containing particles, including Sn and SnSb, during initial cycles of electrochemical lithiation/delithation has been revealed by in situ synchrotron transmission X-ray microscopy, complemented by in situ synchrotron X-ray diffraction to provide phase information. The microstructures and deformation rates are shown to depend on particle composition, size, and alloy stoichiometry, with Li. During first lithiation, both particles exhibit core (metal)-shell (lithiated compounds) interior stractures. Initial formation of a dense surface layer containing LixSn phases of low Li-stoichiometry on the Sn particle hinder further lithiation kinetics, resulting in delayed expansion of large particles. In contrast, Sb in SnSb is readily lithiatecl to form a porous Li-rich (Li3Sb) surface layer it higher potential than Sn, which enables the acceleration of lithiation and removal of the size dependence of the lithiation process. Both lithiated particles only partially contract upon delithiation, and their interiors evolve into porous structures due to metal recrystallization. Such porous structures allow for fast lithiation and mitigated dimensional variations upon subsequent cycles. Neither of the two anode particles pulverize upon cycling.